For years transport-category commercial aircraft have utilized hydraulic brakes. Transport-category aircraft are required by law (14 CFR 25.735) to have a parking brake system that will prevent the aircraft from rolling in the most adverse condition of maximum thrust on one engine and up to maximum ground idle thrust on any or all remaining engines.
One type system that meets this requirement is a mechanical system that couples the operation of a hand brake to the brake pedals by means of a lever and pawl mechanism. This mechanism would lock the brake pedals in place when the hand brake is engaged. The amount of clamping force would therefore be consistent and sufficient to overcome the most adverse condition. Some known disadvantages of this system are that the brake clamped force is always at a maximum which causes wear on the brake and that hydraulic pressure may eventually bleed away, thereby releasing the applied pressure on the brake.
Electrical brakes have since replaced direct hydraulic brakes and are able to provide variable brake clamping force. When the aircraft is at rest, the amount of clamping force required to prevent rolling is minimal. However, when the engines produce thrust, electrical brakes are able to adjust the amount of clamping force to prevent the aircraft from rolling.
Aircraft brakes must also be permanent and non-volatile such that when aircraft power is turned off a locking mechanism will prevent brake clamp force from being removed. Additionally, an interlocking approach must be used so that no single control application flight crew can inadvertently set or inadvertently release the parking brake. Furthermore, no single failure of any component or wire should allow the parking brake function to be activated or deactivated. Electrical brake control systems utilize a locking lever, similar to that used in hydraulic braking systems, for two reasons: the lever provides a familiarity to pilots who are used to operating hydraulic brake systems, and the lever provides a non-volatile state storage and indication for the electrical brake system. The actuators on the electric brake itself contain frictional locking devices which, when set, mechanically lock the actuator in a specific position such that the brake clamping force is retained even after aircraft and braking electrical power are removed.
Park Brake Lever systems require the design of a device to act like the lever and pawl mechanism of hydraulic braking systems. This system is of limited reliability because of its mechanical design which may lead to failures. Mechanical wear or failure of the mechanical switches may cause the lever to indicate a false positive, a false negative, or may be placed in a mode where lever position is out of sync with the parking function.
One type of parking brake mechanism known in the art includes a park brake lever module that includes a lever, two micro switches and a solenoid. The lever includes a stop tab for defining the full travel of the lever, a locking tab that keeps the lever from being inadvertently raised, and two switch actuator tabs that are used to actuate and keep actuated the two micro switches. One micro switch is referred to as the Not Release Switch that indicates “Release” when not activated and “Not Release” when actuated. The second micro switch is referred to as the Set Switch and indicates “Not Set” when not actuated and “Set” when actuated. As the lever is moved it engages the “Not Release Switch.” If the pilot then depresses the brake pedals, the combination of pedal input and actuation of the “Not Release Switch” activates a solenoid, which allows the lever to continue to travel to its full extent. At full extent, the “Set Switch” indicates that the parking brake is set. Releasing the pedals will allow the unlocking solenoid to lock the lever in the up position. Depressing the pedals again (with the “Not Release Switch” closed) will activate the unlock solenoid which then will allow free movement of the lever so that it can be lowered, thus disengaging the parking brake function.
The braking system may include a variety of modes, such as the Engine Run Park Mode, Park and Adjust Mode and Normal Brake Mode. Each of these modes may apply a different level of frictional braking depending on the amount of thrust applied to the aircraft. In Park and Adjust Mode, for example, the friction brakes may apply a minimum of 25% clamping force. In Engine Run Park Mode the friction brakes may apply up to 100% of clamping force.
Therefore, there exists a need in the art for an aircraft parking brake system that eliminates mechanical elements.
There further exists a need in the art for an aircraft parking brake system that utilizes a dual-input to prevent accidental engagement of the parking brake system.
There further exists a need in the art for an aircraft parking brake system that eliminates the potential for false positives and negatives.